Hybrid electric vehicles (HEVs) refer to vehicles that use two or more different kinds of power sources. In general, the HEVs are driven by an engine, which generates driving power by combusting a fuel, and a motor, which generates driving power using electric energy of a battery.
FIG. 1 is an illustrative view of a powertrain system for HEVs, particularly a transmission-mounted electric device (TMED)-type powertrain system, in which a driving motor 3 and a transmission 4 are directly connected to each other.
As shown in the drawing, in the TMED powertrain system, the transmission 4 is mounted to an output side of the driving motor 3 for vehicle traveling so that the output shaft of the motor is connected to the input shaft of the transmission, and accordingly, the speed of the motor becomes the input speed of the transmission.
Specifically, a TMED hybrid electric vehicle includes an engine 1 and a motor 3, which are power sources for driving the vehicle, an engine clutch 2 disposed between the engine 1 and the motor 3, a transmission 4 connected to the output side of the motor 3, an inverter 5 for driving the motor 3, and a battery 6, which serves as a power source (an electric power source) for the motor 3 and is connected to the motor 3 via the inverter 5 for charging or discharging.
Reference numeral 7 in FIG. 1 denotes a hybrid starter and generator (HSG), which is a kind of motor, connected to the engine 1 for transmitting power for starting the engine or generating electric power using rotational force transmitted from the engine.
The HSG 7 operates as a motor or a generator, and is connected to the engine via a power transmission device, such as a belt, a pulley, etc., such that power can be transmitted at all times.
The engine clutch 2 transmits or interrupts power between the engine 1 and the motor 3 through an engagement (closing) operation or disengagement (opening) operation thereof. The inverter 5 converts direct current of the battery 6 into three-phase alternating current and applies the same to the motor 3 to drive the motor 3 and the HSG 7.
The transmission 4 transmits the power of the motor 3 or the combined power of the engine 1 and the motor 3 to driving wheels while performing gear shifting. In hybrid electric vehicles, the transmission may be embodied as an automatic transmission (AT) or a dual-clutch transmission (DCT).
The hybrid electric vehicle having the above construction may be driven in an electric-vehicle (EV) mode, which is a pure electric-vehicle mode using only the power of the motor 3, or a hybrid-electric-vehicle (HEV) mode, which uses the power of both the engine 1 and the motor 3.
Further, when the vehicle is braking or coasting by inertia, it may be driven in a regenerative-braking mode for charging the battery (charged by the motor) by collecting kinetic energy of the vehicles through the motor 3.
In the regenerative-braking mode, the motor 3, which receives the kinetic energy of the vehicles, generates electric power and charges the battery 6, which is connected thereto via the inverter 5.
Along with this operation, when the HSG 7 operates as a generator, it charges the battery 6 via the inverter 5.
Meanwhile, in order to transfer between the EV mode and the HEV mode, the engine clutch 2, which transmits or interrupts power between the engine 1 and the motor 3, is controlled so as to be converted into an engaged (closed) state or a disengaged (open) state.
For example, when the driving mode is switched from the EV mode to the HEV mode, the speed of the engine 1 and the speed of the motor 3 are synchronized, and the engine clutch 2 is engaged after the synchronization, whereby torque variation is prevented from occurring in the process of transmitting power between the two different power sources, namely the engine 1 and the motor 3, and consequently, drivability can be secured.
Specifically, when the driving mode is switched from the EV mode to the HEV mode, after the engine cranking is performed by the HSG 7, the speeds of the two opposite ends of the engine clutch 2, that is, the speed of the engine 1 and the speed of the motor 3, are controlled such that the speed difference therebetween is reduced below a predetermined value, and subsequently, slip control is performed with respect to the engine clutch 2.
When a predetermined period of time has elapsed since the slip control was performed with respect to the engine clutch 2, it may be determined that the speed of the engine 1 and the speed of the motor 3 have been synchronized, and subsequently, the engine clutch 2 may be completely engaged and thereby completes the conversion into the HEV mode.
Such an engagement/disengagement operation of the engine clutch 2 is achieved by a hydraulic control actuator, which is controlled in accordance with a control command from a controller.
Further, the engagement/disengagement operation of the engine clutch 2 is achieved by supplying hydraulic pressure or stopping the supply of hydraulic pressure to a concentric slave cylinder (CSC) using a motor of the hydraulic control actuator.
The engine clutch 2 may be classified into a wet engine clutch and a dry engine clutch. Most hybrid electric vehicles employ a normally-closed-type dry clutch, which uses the aforementioned hydraulic control actuator, as an engine clutch.
Meanwhile, a working fluid, which is used for the engagement (closing) or disengagement (opening) operation of the engine clutch, is characterized in that a volume of the working fluid varies with ambient temperature. Specifically, as the temperature decreases, the volume of the working fluid decreases, and as the temperature increases, the volume of the working fluid increases.
Therefore, if variation in the volume of the working fluid that varies with temperature is not normally detected and the constituent components are not properly controlled corresponding to the variation, the engine clutch may undesirably undergo engagement or disengagement operation, which may cause physical damage to the engine clutch.
Further, when variation in the volume of the working fluid due to a sudden change in temperature occurs, responsiveness to a control command with respect to the stroke of the hydraulic control actuator and accuracy of the stroke of the concentric slave cylinder and/or the engine clutch with may be deteriorated.
FIG. 2 is an illustrative view of the problems shown in the prior art, which illustrates the states of a master cylinder, a concentric slave cylinder and an engine clutch in accordance with variations in temperature and volume of the working fluid.
FIG. 2A illustrates an open (disengaged) state of the engine clutch 2, in which a motor (not shown) is driven to make the master cylinder 8 generate hydraulic pressure and in which the piston 9a of the concentric slave cylinder 9 is moved by the hydraulic pressure.
In particular, the state of the engine clutch shown in FIG. 2A is the normally controlled state. In other words, in the normal case, the engine clutch 2 is controlled so as to be switched to the state shown in FIG. 2A in response to a control command with respect to the actuator.
If the temperature of the working fluid is relatively low, as shown in FIG. 2B, the volume of the working fluid decreases, and the engine clutch 2 is pushed less compared to the case of normal operation by the piston 9a of the concentric slave cylinder 9, whereby the engine clutch 2 is switched to a slip state or a closed (engaged) state, rather than the normal state in FIG. 2A, which may cause damage to the engine clutch.
Conversely, if the temperature of the working fluid is relatively high, as shown in FIG. 2C, the volume of the working fluid increases, and the engine clutch 2 is pushed further than in the case of normal operation by the piston 9a of the concentric slave cylinder 9, whereby the engine clutch 2 is switched to an excessively open state, which is wider than the normal state in FIG. 2A.
However, hydraulic pressure compensation control and actuator control technologies, which enable the engagement/disengagement operation of the engine clutch to be accurately performed in consideration of variation in the volume of the working fluid attributable to variation in temperature, have not been developed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.